We shall develop a method for separating human peripheral blood mononuclear leucocytes into different subpopulations by methods which will recover cells in high yield and purity and which will not impair their functional integrity. Reagents which have high affinities for specific cell surface components will be covalently bound to iron containing microspheres. These will be mixed with heterogeneous populations of leucocytes and the mixtures passed through a magnetic field. The cells which have surface receptors for the reagents attached to the microspheres will be retarded and separated from the remainder of the cells. Sequential separations will yield B lymphocytes (and B lymphocyte subpopulations), T lymphocytes (Fc rec plus and Fc rec-) and monocytes. Cells obtained in this manner will be characterized with respect to surface markers and functional integrity. The acquisition of these purified cell populations will allow studies of the roles of specific cell types in immune functions, specific cell interactions essential to certain immune functions, and assessment of the numbers and functional integrity of specific cell types important to immune function in patients with neoplastic diseases.